1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device such as a reverse-blocking three-terminal thyristor (referred to as "SCR" hereinafter) and the like.
2. Description of the Background Art
In a semiconductor device such as an SCR and the like, a plurality of pn junctions are formed in a semiconductor substrate, and at least one of the pn junctions is exposed on a side surface of the semiconductor substrate. Thus, when the pn junction is exposed on the side surface of the semiconductor substrate, a concentration of electric field in a part of the exposed pn junctions causes withstand voltage of an element to decline, and therefore, the side surface of the semiconductor substrate where the pn junction is exposed is processed to have a bevel structure.
FIG. 3 is a sectional view showing an SCR manufactured by a prior art of manufacturing method to have a double positive bevel structure. Now, steps of manufacturing the SCR will be described with reference to FIG. 2.
First, a pnpn structure composed of three pn functions is formed in a silicon (Si) substrate 1, and thereafter, a circumferential part of the Si substrate 1 is cut. In this way, there are two of the pn junctions exposed in the side surface of the Si substrate 1.
Then, the Si substrate 1 is fixed to a top surface of a metal plate 3 made of molybdenum (Mo) or wolfram (W) by soldering with an aluminum layer 2. The metal plate 3 serves as an anode electrode plate and also as a temperature compensating plate. Further, after a gate electrode layer 4a and a cathode electrode layer 4b are selectively formed on the top major surface of the Si substrate 1, the side surface of the Si substrate having the pn junctions exposed is processed by sandblasting so as to have a double positive bevel structure.
On processing to obtain the bevel structure, many minute faults such as fissures, cracks and the like are caused in the side surface of the Si substrate 1, and hence, the side surface of the Si substrate 1 is spin-etched with an etchant which consists of hydrofluoric acid (HF) mixed with nitric acid (HNO.sub.3). Because of the etching, the side surface of the Si substrate 1 is removed by approximately 20 .mu.m to eliminate the faults, and consequently, the side surface of the Si substrate 1 is flattened and cleaned as well.
After that, the side surface of the Si substrate 1 is cleaned in pure water and dehydrated in organic solvent such as acetone and the like. Then, a varnished layer 5 for protection and an insulating layer 6 of silicone rubber or the like are formed on the side surface of the Si substrate 1. After steps of packaging and the like, the SCR having the double positive bevel structure can be obtained.
Recently in a field of semiconductor devices for electric power, a semiconductor device of withstand voltage as much as thousands of volts has been developed. With such semiconductor devices, enhancing voltage resistance and reliability is especially important. However, in a semiconductor device obtained by the above-mentioned prior manufacturing method, there arises the problem that a decline of withstand voltage in a high electric field is caused.
For example, on etching the side surface of the Si substrate 1 processed in the bevel structure, the metal plate 3 of Mo, W or the like is simultaneously etched away. Because of this, as shown in FIG. 3, an oxide film 7 containing much heavy metal like Mo or W is deposited on the side surface of the Si substrate 1. This phenomenon is well known. When the exposed surface of the pn junctions is covered with such an oxide film 7, leak current through the heavy metal in the oxide film 7 increases. As a result, the decline of withstand voltage of an element in a high electric field, as previously mentioned, is caused.